2007 Vox AC30HH random crackle when shutting off

[gui_tarzan]

I have this head on my bench right now and the owner said it's crackling on occasion when he shuts it off. If it sits for a couple of weeks it doesn't do it but if it's played for several hours it may or may not make this noise when powering off. It's random and I don't see any visual indications of bad caps. I'm leaning toward it being the sound of a cap discharging but I could be wrong. I don't know where this is coming from. I cleaned the tube sockets with DeOxit just in case and it sounds very nice.

Any ideas?

[gui_tarzan]

Nobody?

[nickb]

Nobody?

Yup, we're clueless

The first step in debugging is try to find a way to make it happen. Heat, cool, rattle, shake, prod.. If you can't do that you'll never know if you fixed it. Once you have it happening isolate it e.g. changing control position, pulling tubes, connectors... but I suspect you already knew that, didn't you?

Edit : Updated link. This a good book and a handy poster Download the Poster | Debugging Rules!

[Enzo]

No, the first step is to decide, "Is this actually a problem or just something we notice?"

EVer drive a long way on a warm day in your car, then turn it off get out, and for the next couple minutes little ticks and noises come out of it here and there? Thermal expansion and contraction. I have no trouble thinking occasionally thermal contraction in a tube could make audible noise.

How long does the noise last?

[gui_tarzan]

Nick, I am trying those types of things.

Enzo, I don't know because I haven't heard it, but the way he stated it it sounded like it was maybe a second or two? What's weird is that it only happens after it's used for a period of time, like several hours. When it sits for a couple of weeks or more it doesn't make any noise when turning it off. I'm going to leave it on all evening to see what happens. It could very well be something simple and common that the guy he loaned it to noticed that maybe another amp he has doesn't do. I'm aware of metal making noise when it cools down, such as you mentioned, I'm wondering if it's just some discharge noise he hears. This doesn't have a stand-by switch and I've heard that "crackle" on amps I've shut of and then turned the stand-by back on, like hearing a cap discharge while there's still a signal going through the amp as you kill the power.

[Mick Bailey]

Does the amp have the grid stoppers on the tube sockets or on the board? Not saying this is the problem, but it needs checking anyhow. If they're on the board the amp can crackle sometimes when played with certain tubes. Given that, there may be a possibility of strange noises as the voltage drains when powering off.

Only early models were affected - later ones had a revision and Korg issued a technical bulletin to re-work older models.

[gui_tarzan]

Good question, I'll check that when I get home tonight. The manufacture tag says "10/2007", I don't know if that was early or late.

[gui_tarzan]

1500 ohm on pin 2 of the EL84s, 220 ohm on the circuit board that go to pin 9 on the EL84s.